Symonds met Aleister Crowley in 1946, the year before Crowley's death. Crowley's will left the copyright of his works to Symonds and made him Crowley's literary executor, though Crowley's legal status as an undischarged bankrupt meant that the copyrights actually ended up in receivership. At first fascinated by Crowley, Symonds became increasingly critical of his ideas and manners, in particular the use of drugs and free-sex.
He edited, with Kenneth Grant, Crowley's autobiography and a number of other works. He authored four biographical works of his own: The Great Beast, (1952), The Magic of Aleister Crowley (1958), The King of the Shadow Realm (1989) and The Beast 666 (1997).